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作者：android超級兵?
https://blog.csdn.net/weixin_44819566

前言

還是老套路,先來看看實現(xiàn)的效果!

在寫這個效果之前,需要熟悉Rv的回收復用機制,因為實現(xiàn)這個效果,需要自定義LayoutManager()…

眾所周知,RecyclerView 是一個可滑動的View,那么他的回收/復用入口一定是在onTouchEvent()事件中

滑動過程中響應的是MotionEvent.ACTION_MOVE事件,所以直接來這里找找看!!

緩存機制

onTouchEvent()入口

#RecyclerView.java

?@Override
public?boolean?onTouchEvent(MotionEvent?e)?{

????final?int?action?=?e.getActionMasked();
?????switch?(action)?{
????????????........................................
???????????????........只展示代碼思路,細節(jié)請自行查看........
???????????????........................................

????????????case?MotionEvent.ACTION_MOVE:?{
????????????if?(mScrollState?==?SCROLL_STATE_DRAGGING)?{
????????????????????mLastTouchX?=?x?-?mScrollOffset[0];
????????????????????mLastTouchY?=?y?-?mScrollOffset[1];

????????????????????//?關(guān)鍵代碼1
????????????????????if?(scrollByInternal(
????????????????????????????canScrollHorizontally???dx?:?0,
????????????????????????????canScrollVertically???dy?:?0,
????????????????????????????vtev))?{
????????????????????????getParent().requestDisallowInterceptTouchEvent(true);
????????????????????}
????????????????????if?(mGapWorker?!=?null?&&?(dx?!=?0?||?dy?!=?0))?{
????????????????????????mGapWorker.postFromTraversal(this,?dx,?dy);
????????????????????}
????????????????}
????????????}
????????????break;
????}
}

接著找scrollByInternal(int x, int y, MotionEvent ev)方法

#RecyclerView.java

boolean?scrollByInternal(int?x,?int?y,?MotionEvent?ev)?{
?????if?(mAdapter?!=?null)?{
????????????........................................
???????????????........只展示代碼思路,細節(jié)請自行查看........
???????????????........................................
????????????if?(x?!=?0)?{
????????????????//?關(guān)鍵代碼2?去到?LinearLayoutManager?執(zhí)行fill方法
????????????????consumedX?=?mLayout.scrollHorizontallyBy(x,?mRecycler,?mState);
????????????????unconsumedX?=?x?-?consumedX;
????????????}
????????????if?(y?!=?0)?{
????????????????//?關(guān)鍵代碼2?去到LinearLayoutManager?執(zhí)行fill方法
????????????????consumedY?=?mLayout.scrollVerticallyBy(y,?mRecycler,?mState);
????????????????unconsumedY?=?y?-?consumedY;
????????????}
????????}
????????....
}

現(xiàn)在走到了mLayout.scrollHorizontallyBy(x, mRecycler, mState);

接著去LinearLayoutManager()?中去找scrollHorizontallyBy()?方法

#LinearLayoutManager.java

????@Override
????public?int?scrollVerticallyBy(int?dy,?RecyclerView.Recycler?recycler,
??????????????????????????????????RecyclerView.State?state)?{
????????if?(mOrientation?==?HORIZONTAL)?{
????????????return?0;
????????}
????????//?關(guān)鍵代碼3
????????return?scrollBy(dy,?recycler,?state);
????}

scrollBy()->

#LinearLayoutManager.java

?int?scrollBy(int?dy,?RecyclerView.Recycler?recycler,?RecyclerView.State?state)?{
???????........................................
?????........只展示代碼思路,細節(jié)請自行查看........
?????........................................
?????final?int?consumed?=?mLayoutState.mScrollingOffset
????????????????//?關(guān)鍵代碼4
????????????????+?fill(recycler,?mLayoutState,?state,?false);
}

接著找到fill()方法

#LinearLayoutManager.java

int?fill(RecyclerView.Recycler?recycler,?LayoutState?layoutState,
?????????????RecyclerView.State?state,?boolean?stopOnFocusable)?{


????????if?(layoutState.mScrollingOffset?!=?LayoutState.SCROLLING_OFFSET_NaN)?{

????????????//?關(guān)鍵代碼19?緩存ViewHolder
????????????recycleByLayoutState(recycler,?layoutState);
????????}

????????//?循環(huán)調(diào)用
????????while?((layoutState.mInfinite?||?remainingSpace?>?0)?&&?layoutState.hasMore(state))?{

???????????//?關(guān)鍵代碼5?[用來4級復用]
????????????layoutChunk(recycler,?state,?layoutState,?layoutChunkResult);

??????????????????........................................
????????????????????........只展示代碼思路,細節(jié)請自行查看........
??????????????????........................................
????????}

????}

看到這里只需要記住以下兩點即可:

recycleByLayoutState(recycler, layoutState); 緩存ViewHolder
layoutChunk(recycler, state, layoutState, layoutChunkResult); 四級復用

有人可能會問,這里為什么是四級?不是說的三級嘛?

其實三級和四級都無所謂,知識點是不會變的,只是層級越多,理解就越深刻,越細罷了

直接進入到緩存的代碼:

#LinearLayoutManager.java

?private?void?recycleByLayoutState(RecyclerView.Recycler?recycler,?LayoutState?layoutState)?{
????????if?(layoutState.mLayoutDirection?==?LayoutState.LAYOUT_START)?{
????????????//?關(guān)鍵代碼21?緩存底部
????????????recycleViewsFromEnd(recycler,?layoutState.mScrollingOffset);
????????}?else?{
????????????//?關(guān)鍵代碼20?緩存頭部
????????????recycleViewsFromStart(recycler,?layoutState.mScrollingOffset);
????????}
????}

這里如果是向下滑動,就會緩存頭部那么就會執(zhí)行到recycleViewsFromStart()

如果是向上滑動,就會緩存尾部那么就會執(zhí)行到recycleViewsFromEnd()

recycleViewsFromStart() 和 recycleViewsFromEnd() 隨便點開一個看看,里面代碼都差不多一樣.

#LinearLayoutManager.java

?private?void?recycleViewsFromStart(RecyclerView.Recycler?recycler,?int?dt)?{

????????if?(mShouldReverseLayout)?{
????????????for?(int?i?=?childCount?-?1;?i?>=?0;?i--)?{
????????????...
????????????????????//?關(guān)鍵代碼22
????????????????????recycleChildren(recycler,?childCount?-?1,?i);
????????????????????return;
????????????}
????????}?else?{
????????????for?(int?i?=?0;?i?????????????...
????????????????????//?關(guān)鍵代碼23
????????????????????recycleChildren(recycler,?0,?i);
????????????????????return;
????????????}
????????}
????}

這里無論走哪一個if()?都會走到recycleChildren()方法

#LinearLayoutManager.java

private?void?recycleChildren(RecyclerView.Recycler?recycler,?int?startIndex,?int?endIndex)?{
????????if?(startIndex?==?endIndex)?{
????????????return;
????????}
????????if?(endIndex?>?startIndex)?{
????????????for?(int?i?=?endIndex?-?1;?i?>=?startIndex;?i--)?{
????????????????//?移除View??關(guān)鍵代碼23?[執(zhí)行到RecyclerView.removeAndRecycleViewAt()]
????????????????removeAndRecycleViewAt(i,?recycler);
????????????}
????????}?else?{
????????????for?(int?i?=?startIndex;?i?>?endIndex;?i--)?{
????????????????removeAndRecycleViewAt(i,?recycler);
????????????}
????????}
????}

接著這里會執(zhí)行到RecyclerView的removeAndRecycleViewAt()方法

#RecyclerView.java

????????//?關(guān)鍵代碼24
????????public?void?removeAndRecycleViewAt(int?index,?Recycler?recycler)?{
????????????final?View?view?=?getChildAt(index);
????????????removeViewAt(index);
????????????//?關(guān)鍵代碼25
????????????recycler.recycleView(view);
????????}

繼續(xù)往下執(zhí)行

#RecyclerView.java

?public?void?recycleView(View?view)?{
????????????.......
????????????ViewHolder?holder?=?getChildViewHolderInt(view);
????????????//?緩存
????????????recycleViewHolderInternal(holder);
????????}

接著繼續(xù)執(zhí)行recycleViewHolderInternal()

#RecyclerView.java

void?recycleViewHolderInternal(ViewHolder?holder)?{
????????????........................................
????????????........只展示代碼思路,細節(jié)請自行查看........
????????????........................................
?????????????boolean?cached?=?false;

????????????if?(forceRecycle?||?holder.isRecyclable())?{
????????????????//?mViewCacheMax?=?緩存的最大值?
????????????????//?mViewCacheMax?=?2
????????????????//?如果viewHolder是無效、未被移除、未被標記的
????????????????if?(mViewCacheMax?>?0
????????????????????????&&?!holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID
????????????????????????|?ViewHolder.FLAG_REMOVED
????????????????????????|?ViewHolder.FLAG_UPDATE
????????????????????????|?ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN))?{
????????????????????int?cachedViewSize?=?mCachedViews.size();

????????????????????//?關(guān)鍵代碼24
????????????????????//?mViewCacheMax?=?2
????????????????????if?(cachedViewSize?>=?mViewCacheMax?&&?cachedViewSize?>?0)?{
????????????????????????//?如果viewholder存滿2個則移除第0個位置?
????????????????????????//?保證mCachedViews?最多能緩存2個ViewHolder
????????????????????????recycleCachedViewAt(0);
????????????????????????cachedViewSize--;
????????????????????}
????????????????????....
????????????????????//?保存ViewHolder數(shù)據(jù)?[mCachedViews數(shù)據(jù)不會超過2個]
????????????????????mCachedViews.add(targetCacheIndex,?holder);
????????????????????cached?=?true;
????????????????}
?????????????????if?(!cached)?{
????????????????????//?當ViewHolder不改變時候(只有一個ViewHolder)?就會直接存到緩存池中
????????????????????addViewHolderToRecycledViewPool(holder,?true);
????????????????????recycled?=?true;
????????????????}
????????????????........................................
???????????????????........只展示代碼思路,細節(jié)請自行查看........
????????????????........................................
????????}

通過?關(guān)鍵代碼24?可知,mCachedViews 最多能保存2個ViewHolder

如果第三個ViewHolder來臨的時候,就會先刪除掉第0個,然后在?mCachedViews.add(targetCacheIndex, holder);

然后再來看看?recycleCachedViewAt(0)的細節(jié)!

#RecyclerView.java

????void?recycleCachedViewAt(int?cachedViewIndex)?{
????????????...
????????????ViewHolder?viewHolder?=?mCachedViews.get(cachedViewIndex);

????????????//?關(guān)鍵代碼25
????????????//?添加到ViewPool到緩存里面取
????????????addViewHolderToRecycledViewPool(viewHolder,?true);

????????????//?將第0個ViewHolder移除
????????????mCachedViews.remove(cachedViewIndex);
????????}

繼續(xù)執(zhí)行到?addViewHolderToRecycledViewPool()方法

將mCachedViews.get(0)中的ViewHolder獲取出來,添加到緩存池中,并刪除

#RecyclerView.java

void?addViewHolderToRecycledViewPool(ViewHolder?holder,?boolean?dispatchRecycled)?{
????????????.....
????????????//?向緩存池中?保存ViewHolder?關(guān)鍵代碼28
????????????getRecycledViewPool().putRecycledView(holder);
????????}

點進來看看putRecycledView()方法

#RecyclerView.java

//?SparseArray?類似與?HashMap
//?特點:?key相同會保留最后一個,
//??????會根據(jù)key的int值排序(從小到大)
SparseArray?mScrap?=?new?SparseArray<>();

?public?void?putRecycledView(ViewHolder?scrap)?{
???????//?獲取ViewHolder布局類型
??????final?int?viewType?=?scrap.getItemViewType();

??????//?根據(jù)布局類型來獲取ViewHolder
???????final?ArrayList?scrapHeap?=?getScrapDataForType(viewType).mScrapHeap;

???????//?判斷緩存池的大小
???????//?mScrap.get(viewType).mMaxScrap?默認為?5
???????//?同一種ViewType?只保存5個ViewHolder
????????if?(mScrap.get(viewType).mMaxScrap?<=?scrapHeap.size())?{
???????????return;
????????}

???????//?清空ViewHolder記錄
????????scrap.resetInternal();

????????//add
????????scrapHeap.add(scrap);
}
?//?清空ViewHolder記錄
?void?resetInternal()?{
????????????mFlags?=?0;
????????????mPosition?=?NO_POSITION;
????????????mOldPosition?=?NO_POSITION;
????????????mItemId?=?NO_ID;
????????????mPreLayoutPosition?=?NO_POSITION;
????????????mIsRecyclableCount?=?0;
????????????mShadowedHolder?=?null;
????????????mShadowingHolder?=?null;
????????????clearPayload();
????????????mWasImportantForAccessibilityBeforeHidden?=?View.IMPORTANT_FOR_ACCESSIBILITY_AUTO;
????????????mPendingAccessibilityState?=?PENDING_ACCESSIBILITY_STATE_NOT_SET;
????????????clearNestedRecyclerViewIfNotNested(this);
????????}

//?根據(jù)不同viewType?獲取ViewHolder
?private?ScrapData?getScrapDataForType(int?viewType)?{
????????????ScrapData?scrapData?=?mScrap.get(viewType);
????????????if?(scrapData?==?null)?{
????????????????scrapData?=?new?ScrapData();
????????????????mScrap.put(viewType,?scrapData);
????????????}
????????????return?scrapData;
????????}

可以看出,緩存池,中最多保存5個同一類型的ViewHolder,并且ViewHolder是空的ViewHolder,

而且緩存池中保存的都是mCachedViews移除的數(shù)據(jù)!

小結(jié)

mCachedViews?保存即將離開屏幕外的2個ViewHolder

mRecyclerPool 緩存池中:同一種ItemViewType類型能夠默認最多保存5個空數(shù)據(jù)的ViewHolder.

帶入實戰(zhàn)看看效果:

這里以單布局(ItemViewType = 0)為例

我的layoutManger為`GridLayoutManager(content,7)`,所以每次劃出屏幕的時候,就直接會劃走7個ViewHolder

可以看出,劃出去的一剎那,前5個不會執(zhí)行onCreateViewHolder(),后2個會執(zhí)行onCreateViewHolder()

??:onCreateViewHolder()?是用來創(chuàng)建ViewHolder的,后面復用的時候會說!

走到這里,只是分析了RecyclerView從onTouchEvent()–>MOVE事件滑動事件

最終會把ViewHolder保存mCachedViews, mCachedViews只能保存2個ViewHolder

如果第三個ViewHolder來臨的時候,就保存到緩存池(mRecyclerPool)中

緩存池(mRecyclerPool)最多保存5個空的ViewHolder…

這只是一種緩存的入口,緩存還有另一種入口,在RecyclerView 的 onLayout()的時候

mAttachedScrap和mChangedScrap 會緩存屏幕內(nèi)可見的ViewHolder

onLayout()入口

????#RecyclerView.java
????@Override
????protected?void?onLayout(boolean?changed,?int?l,?int?t,?int?r,?int?b)?{
????????//?入口
????????dispatchLayout();
????}

接著執(zhí)行dispatchLayout()

#RecyclerView.java

void?dispatchLayout()?{
???????.....
???????dispatchLayoutStep2();
???????......
}

接著執(zhí)行dispatchLayoutStep2()

#RecyclerView.java

private?void?dispatchLayoutStep2()?{
??????????......

????????//?在這里先緩存
????????mLayout.onLayoutChildren(mRecycler,?mState);
????????.....
}

接著走到LinearLayoutManager.onLayoutChildren()方法

#LinearLayoutManager.java

?@Override
public?void?onLayoutChildren(RecyclerView.Recycler?recycler,?RecyclerView.State?state)?{
????????....
????????//會執(zhí)行到:?RecyclerView.detachAndScrapAttachedViews()
?????????detachAndScrapAttachedViews(recycler);
????????......
}

這里會走到RecyclerView.detachAndScrapAttachedViews(),這行代碼非常關(guān)鍵,可以說是緩存屏幕內(nèi)的ViewHolder的起點,后面完成”探探“效果也需要用到!!

#RecyclerView.java

public?void?detachAndScrapAttachedViews(Recycler?recycler)?{
????????????final?int?childCount?=?getChildCount();
????????????for?(int?i?=?childCount?-?1;?i?>=?0;?i--)?{
????????????????final?View?v?=?getChildAt(i);
????????????????//?回收機制關(guān)鍵代碼1
????????????????scrapOrRecycleView(recycler,?i,?v);
????????????}
}

繼續(xù)走scrapOrRecycleView()

#RecyclerView.java

private?void?scrapOrRecycleView(Recycler?recycler,?int?index,?View?view)?{
????????????final?ViewHolder?viewHolder?=?getChildViewHolderInt(view);
???????????...
????????????if?(viewHolder.isInvalid()?&&?!viewHolder.isRemoved()
????????????????????&&?!mRecyclerView.mAdapter.hasStableIds())?{
????????????????removeViewAt(index);
????????????????//?緩存機制關(guān)鍵代碼2?主要用來處理?cacheView?,RecyclerViewPool的緩存
????????????????recycler.recycleViewHolderInternal(viewHolder);
????????????}?else?{
????????????????detachViewAt(index);
????????????????//?緩存機制關(guān)鍵代碼3
????????????????recycler.scrapView(view);
????????????}
}

這里有兩個非常關(guān)鍵的點

緩存機制關(guān)鍵代碼2 主要用來處理 cacheView ,RecyclerViewPool的緩存recycler.recycleViewHolderInternal(viewHolder); // 這個關(guān)鍵點上面已經(jīng)分析過了!!,忘記的ctrl+F搜索看看看一看
recycler.scrapView(view); // 緩存屏幕內(nèi)的ViewHolder

這里直接看看recycler.scrapView(view);的細節(jié)

?void?scrapView(View?view)?{
????????????final?ViewHolder?holder?=?getChildViewHolderInt(view);
????????????//?如果標記沒有移除,或者失效等清空?就會緩存
????????????if?(holder.hasAnyOfTheFlags(ViewHolder.FLAG_REMOVED?|?ViewHolder.FLAG_INVALID)
????????????????????||?!holder.isUpdated()?||?canReuseUpdatedViewHolder(holder))?{
????????????????holder.setScrapContainer(this,?false);
????????????????//?一級緩存位置點1
????????????????mAttachedScrap.add(holder);
????????????}?else?{
????????????????if?(mChangedScrap?==?null)?{
????????????????????mChangedScrap?=?new?ArrayList();
????????????????}
????????????????holder.setScrapContainer(this,?true);
????????????????//?一級緩存位置點2
????????????????mChangedScrap.add(holder);
????????????}
}

走到這里4級緩存就結(jié)束了

總結(jié)一下:

參考深入理解Android RecyclerView的緩存機制：https://segmentfault.com/a/1190000040421118

復用機制

回到fill()方法。ctrl + F搜索一下,上邊說過

#LinearLayoutManager.java

int?fill(RecyclerView.Recycler?recycler,?LayoutState?layoutState,
?????????????RecyclerView.State?state,?boolean?stopOnFocusable)?{

????????final?int?start?=?layoutState.mAvailable;
????????if?(layoutState.mScrollingOffset?!=?LayoutState.SCROLLING_OFFSET_NaN)?{
????????????.....
????????????//?關(guān)鍵代碼19?[用來4級緩存]
????????????recycleByLayoutState(recycler,?layoutState);
????????}
?????????....

????????//?循環(huán)調(diào)用
????????while?((layoutState.mInfinite?||?remainingSpace?>?0)?&&?layoutState.hasMore(state))?{

???????????//?關(guān)鍵代碼5?[用來4級復用]
????????????layoutChunk(recycler,?state,?layoutState,?layoutChunkResult);

??????????????????........................................
????????????????????........只展示代碼思路,細節(jié)請自行查看........
??????????????????........................................
????????}
}

緩存是進入的recycleByLayoutState(recycler, layoutState);方法

復用是進入的layoutChunk()方法

執(zhí)行到layoutState.next(recycler);方法

#LinearLayoutManager.java

void?layoutChunk(RecyclerView.Recycler?recycler,?RecyclerView.State?state,
?????????????????????LayoutState?layoutState,?LayoutChunkResult?result)?{
????????//?獲取當前view
????????//?關(guān)鍵代碼6
????????View?view?=?layoutState.next(recycler);

????????//?測量View
????????measureChildWithMargins(view,?0,?0);
????????.....
}

接著執(zhí)行到recycler.getViewForPosition(mCurrentPosition);

#LinearLayoutManager.java

View?next(RecyclerView.Recycler?recycler)?{
???????????.....
????????????//?關(guān)鍵代碼7?[復用機制入口]
????????????final?View?view?=?recycler.getViewForPosition(mCurrentPosition);
????????????return?view;
}

然后繼續(xù)執(zhí)行到getViewForPosition()–>?getViewForPosition()

#RecyclerView.java

?public?View?getViewForPosition(int?position)?{
????????????//?關(guān)鍵代碼8
????????????return?getViewForPosition(position,?false);
}

View?getViewForPosition(int?position,?boolean?dryRun)?{
?????//?關(guān)鍵代碼10?所有的復用都在這里

??????return?tryGetViewHolderForPositionByDeadline(position,?dryRun,?FOREVER_NS).itemView;
}

最終會執(zhí)行到tryGetViewHolderForPositionByDeadline(),所有的復用代碼都在這里了!

#RecyclerView.java

??ViewHolder?tryGetViewHolderForPositionByDeadline(int?position,
?????????????????????????????????????????????????????????boolean?dryRun,?long?deadlineNs)?{
?????????????ViewHolder?holder?=?null;

????????????//?一級別復用?[mChangedScrap]
????????????if?(mState.isPreLayout())?{
????????????????//?關(guān)鍵代碼11
????????????????holder?=?getChangedScrapViewForPosition(position);
????????????????fromScrapOrHiddenOrCache?=?holder?!=?null;
????????????}

????????????//?一級復用?[mAttachedScrap]
????????????if?(holder?==?null)?{
????????????????//?通過位置
????????????????//?關(guān)鍵代碼12
????????????????holder?=?getScrapOrHiddenOrCachedHolderForPosition(position,?dryRun);

????????????}

????????????//?二級復用?[mCachedViews]
?????????????if?(holder?==?null)?{
????????????????????//?獲取布局類型
????????????????????final?int?type?=?mAdapter.getItemViewType(offsetPosition);

????????????????????//?2)?Find?from?scrap/cache?via?stable?ids,?if?exists
????????????????????//?2)?通過穩(wěn)定ID從廢料/緩存中查找（如果存在）
????????????????????if?(mAdapter.hasStableIds())?{
????????????????????//?關(guān)鍵代碼13?根據(jù)Id來復用
????????????????????holder?=?getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition),
????????????????????????????type,?dryRun);
????????????????????}
?????????????}

?????????????//?三級復用?【自定義復用】
????????????????if?(holder?==?null?&&?mViewCacheExtension?!=?null)?{
????????????????????//?關(guān)鍵代碼14
????????????????????//?自定義復用
????????????????????final?View?view?=?mViewCacheExtension
????????????????????????????.getViewForPositionAndType(this,?position,?type);
????????????????????if?(view?!=?null)?{
????????????????????????holder?=?getChildViewHolder(view);
?????????????????????}
????????????????}

????????????//?四級復用?[mRecyclerPool(緩存池復用)]
??????????????if?(holder?==?null)?{
????????????????????//?關(guān)鍵代碼15?從緩存池獲取viewHolder
????????????????????holder?=?getRecycledViewPool().getRecycledView(type);

????????????????}

????????????//?最終,如果走到這里,holder?==?0,表示沒有緩存,那么則創(chuàng)建ViewHolder
????????????if?(holder?==?null)?{
????????????????????//?如果四級緩存都是?null,?那么就由適配器創(chuàng)建?ViewHolder
????????????????????holder?=?mAdapter.createViewHolder(RecyclerView.this,?type);
????????????}

????????????//?走到這了的時候,ViewHolder?!=?null
????????????//?綁定布局
????????????if?(mState.isPreLayout()?&&?holder.isBound())?{
???????????????.....
????????????}?else?if?(!holder.isBound()?||?holder.needsUpdate()?||?holder.isInvalid())?{
?????????????????......
????????????????//?關(guān)鍵代碼17
????????????????//?在這里調(diào)?onBindViewHolder()?綁定數(shù)據(jù)
????????????????bound?=?tryBindViewHolderByDeadline(holder,?offsetPosition,?position,?deadlineNs);
????????????????......
????????????}
????????????......
}

看一下tryBindViewHolderByDeadline(),綁定ViewHolder的具體綁定細節(jié):

private?boolean?tryBindViewHolderByDeadline(ViewHolder?holder,?int?offsetPosition,
????????????????????????????????????????????????????int?position,?long?deadlineNs)?{
???????????....
???????????//?最終綁定位置
????????????mAdapter.bindViewHolder(holder,?offsetPosition);
???????????...
}

復用機制比緩存機制簡單很多,因為復用入口就一個?？纯戳鞒虉D一目了然!

探探效果實戰(zhàn)

??:為了全局性考慮,實戰(zhàn)采用java,底部附 java/kotlin 源碼

要想實戰(zhàn),那就得先實現(xiàn)最普通的效果,這段代碼沒啥營養(yǎng),直接看效果!

自定義LayoutManager

public?class?CardStack3LayoutManager?extends?RecyclerView.LayoutManager?{
????@Override
????public?RecyclerView.LayoutParams?generateDefaultLayoutParams()?{
?????????return?new?RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,
????????????????ViewGroup.LayoutParams.WRAP_CONTENT);
????}

?????//?必須重寫?在?RecyclerView->OnLayout()時候調(diào)用,用來擺放?Item位置
?????@Override
????public?void?onLayoutChildren(RecyclerView.Recycler?recycler,?RecyclerView.State?state)?{
????????super.onLayoutChildren(recycler,?state);
????}
}

需要重寫generateDefaultLayoutParams()方法,咋們是仿造著 LinearLayoutManager()來寫,所以直接參考 LinearLayoutManager()就可以

注意:這里的?onLayoutChildren()?需要手動重寫!

主要功能都在onLayoutChildren()中編寫

#CardStack2LayoutManager.java

?//?最開始顯示個數(shù)
?public?static?final?int?MAX_SHOW_COUNT?=?4;

????@Override
????public?void?onLayoutChildren(RecyclerView.Recycler?recycler,?RecyclerView.State?state)?{
????????super.onLayoutChildren(recycler,?state);

?????????//?調(diào)用RecyclerView的緩存機制?緩存?ViewHolder
????????detachAndScrapAttachedViews(recycler);

????????//?最下面圖片下標
????????int?bottomPosition?=?0;
????????//?獲取所有圖片
????????int?itemCount?=?getItemCount();

????????if?(itemCount?>?MAX_SHOW_COUNT)?{
????????????//?獲取到從第幾張開始
????????????bottomPosition?=?itemCount?-?MAX_SHOW_COUNT;
????????}

?????????for?(int?i?=?bottomPosition;?i?????????????//?獲取當前view寬高
????????????View?view?=?recycler.getViewForPosition(i);

????????????addView(view);

????????????//?測量
????????????measureChildWithMargins(view,?0,?0);

//????????????getWidth()?RecyclerView?寬
//????????????getDecoratedMeasuredWidth()?View的寬
????????????int?widthSpace?=?getWidth()?-?getDecoratedMeasuredWidth(view);
????????????int?heightSpace?=?getHeight()?-?getDecoratedMeasuredHeight(view);

????????????//?LinearLayoutManager#layoutChunk#layoutDecoratedWithMargins
????????????//?繪制布局
????????????layoutDecoratedWithMargins(view,?widthSpace?/?2,
????????????????????heightSpace?/?2,
????????????????????widthSpace?/?2?+?getDecoratedMeasuredWidth(view),
????????????????????heightSpace?/?2?+?getDecoratedMeasuredHeight(view));
????????????}
}

這段代碼就是獲取所有的 ItemView,然后全部布局到屏幕中心

先來看看當前的效果:

detachAndScrapAttachedViews()上面提到過,是緩存的入口,會直接調(diào)用到RecyclerView.detachAndScrapAttachedViews()方法

測量布局,擺放的代碼參考自 LinearLayoutManager(),思路就是吧當前View添加到RecyclerView中,然后在測量View,最后在擺放(布局)View

最后讓View擺放時候有縮放層級:

#CardStack2LayoutManager.java

?//?最開始顯示個數(shù)
????public?static?final?int?MAX_SHOW_COUNT?=?4;

????//?item?平移Y軸距
????public?static?final?int?TRANSLATION_Y?=?20;

????//?縮放的大小
????public?static?final?float?SCALE?=?0.05f;

@Override
????public?void?onLayoutChildren(RecyclerView.Recycler?recycler,?RecyclerView.State?state)?{
????????super.onLayoutChildren(recycler,?state);

????????//?緩存?ViewHolder
????????detachAndScrapAttachedViews(recycler);

????????//?最下面圖片下標
????????int?bottomPosition?=?0;
????????//?獲取所有圖片
????????int?itemCount?=?getItemCount();

????????//如果所有圖片?>?顯示的圖片
????????if?(itemCount?>?MAX_SHOW_COUNT)?{
????????????//?獲取到從第幾張開始
????????????bottomPosition?=?itemCount?-?MAX_SHOW_COUNT;
????????}

????????for?(int?i?=?bottomPosition;?i?????????????//?獲取當前view寬高
????????????View?view?=?recycler.getViewForPosition(i);

????????????addView(view);

????????????//?測量
????????????measureChildWithMargins(view,?0,?0);

//????????????getWidth()?RecyclerView?寬
//????????????getDecoratedMeasuredWidth()?View的寬
????????????int?widthSpace?=?getWidth()?-?getDecoratedMeasuredWidth(view);
????????????int?heightSpace?=?getHeight()?-?getDecoratedMeasuredHeight(view);


????????????//?LinearLayoutManager#layoutChunk#layoutDecoratedWithMargins
????????????//?繪制布局
????????????layoutDecoratedWithMargins(view,?widthSpace?/?2,
????????????????????heightSpace?/?2,
????????????????????widthSpace?/?2?+?getDecoratedMeasuredWidth(view),
????????????????????heightSpace?/?2?+?getDecoratedMeasuredHeight(view));

????????????/*
?????????????*?作者:android?超級兵
?????????????*?TODO?itemCount?-?1??=?最后一個元素
????????????????????最后一個元素?-?i?=?倒數(shù)的元素
?????????????*/
????????????int?level?=?itemCount?-?1?-?i;

????????????if?(level?>?0)?{
????????????????int?value?=?toDip(view.getContext(),?TRANSLATION_Y);

????????????????//?如果不是最后一個才縮放
????????????????if?(level?1)?{

????????????????????//?平移
????????????????????view.setTranslationY(value?*?level);
????????????????????//?縮放
????????????????????view.setScaleX(1?-?SCALE?*?level);
????????????????????view.setScaleY(1?-?SCALE?*?level);
????????????????}?else?{
????????????????????//?最下面的View?和前一個View布局一樣(level?-?1)
????????????????????view.setTranslationY(value?*?(level?-?1));
????????????????????view.setScaleX(1?-?SCALE?*?(level?-?1));
????????????????????view.setScaleY(1?-?SCALE?*?(level?-?1));
????????????????}
????????????}
????????}
????}

????private?int?toDip(Context?context,?float?value)?{
????????return?(int)?TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,?value,?context.getResources().getDisplayMetrics());
????}

當前效果為:

到目前為止,完成了ItemView的疊加擺放,接下來只需要添加上滑動即可!

RecyclerView拖拽滑動需要使用到ItemTouchHelper.SimpleCallback

public?class?SlideCardStackCallBack2<T>?extends?ItemTouchHelper.SimpleCallback?{
????private?final?CardStackAdapter?mAdapter;

????public?SlideCardStackCallBack2(CardStackAdapter?mAdapter)?{
????????super(0,?15);
????????this.mAdapter?=?mAdapter;
????}

????//?拖拽使用,不用管
????@Override
????public?boolean?onMove(@NonNull?RecyclerView?recyclerView,?@NonNull?RecyclerView.ViewHolder?viewHolder,?@NonNull?RecyclerView.ViewHolder?target)?{
????????return?false;
????}

???//?滑動結(jié)束后的處理
????@Override
????public?void?onSwiped(@NonNull?RecyclerView.ViewHolder?viewHolder,?int?direction)?{

????}
}

這里需要傳遞兩個參數(shù):

參數(shù)一:dragDirs 拖拽
參數(shù)二:swipeDirs 滑動

這里咋們不用拖拽,直接給0就行,主要說一下滑動swipeDirs

#ItemTouchHelper.java

/**
?????*?Up?direction,?used?for?swipe?&?drag?control.
?????*/
????public?static?final?int?UP?=?1;????//1

????/**
?????*?Down?direction,?used?for?swipe?&?drag?control.
?????*/
????public?static?final?int?DOWN?=?1?<1;?//2?

????/**
?????*?Left?direction,?used?for?swipe?&?drag?control.
?????*/
????public?static?final?int?LEFT?=?1?<2;?//4

????/**
?????*?Right?direction,?used?for?swipe?&?drag?control.
?????*/
????public?static?final?int?RIGHT?=?1?<3;?//8?

滑動主要以這幾個位運算組

如果需要上下滑動那么就是 UP+DOWN = 1+2 = 3
如果是上下左滑動就是 UP + DOWN + LEFT = 1 + 2 + 4 = 7
那么如果是上下左右滑動就是 UP + DOWN + LEFT + RIGHT = 15

所以這里直接填15就表示可以上下左右滑動

onSwiped()處理:

#SlideCardStackCallBack2.java

@Override
????public?void?onSwiped(@NonNull?RecyclerView.ViewHolder?viewHolder,?int?direction)?{
????????//?當前滑動的View下標
????????int?layoutPosition?=?viewHolder.getLayoutPosition();
????????//?刪除當前滑動的元素
????????CardStackBean?bean?=?mAdapter.getData().remove(layoutPosition);

????????//?添加到集合第0個位置?造成循環(huán)滑動的效果
????????mAdapter.addData(0,?bean);
????????mAdapter.notifyDataSetChanged();
}

這段代碼很好理解,先刪除當前滑動的View,然后在添加到最后一個,造成循環(huán)滑動的效果!來看看效果:

現(xiàn)在看來,還是有點生硬,添加一些滑動系數(shù)縮放:

這里直接貼出完整代碼:

看圖說話:

#SlideCardStackCallBack2.java

@Override
????public?void?onChildDraw(@NonNull?Canvas?c,?@NonNull?RecyclerView?recyclerView,?@NonNull?RecyclerView.ViewHolder?viewHolder,?float?dX,?float?dY,?int?actionState,?boolean?isCurrentlyActive)?{
????????super.onChildDraw(c,?recyclerView,?viewHolder,?dX,?dY,?actionState,?isCurrentlyActive);
????????int?maxDistance?=?recyclerView.getWidth()?/?2;

????????????//?dx?=?當前滑動x位置
????????????//?dy?=?當前滑動y位置
????????//sqrt?開根號
????????double?sqrt?=?Math.sqrt((dX?*?dX?+?dY?*?dY));

????????//?放大系數(shù)
????????double?scaleRatio?=?sqrt?/?maxDistance;

????????//?系數(shù)最大為1?
????????if?(scaleRatio?>?1.0)?{
????????????scaleRatio?=?1.0;
????????}

????????int?childCount?=?recyclerView.getChildCount();
????????//?循環(huán)所有數(shù)據(jù)
????????for?(int?i?=?0;?i?????????????View?view?=?recyclerView.getChildAt(i);

????????????int?valueDip?=?toDip(view.getContext(),?20f);

????????????/*
?????????????*?作者:android?超級兵
?????????????*?TODO
?????????????*???childCount?-?1?=??itemView總個數(shù)
?????????????*????childCount?-?1?-?i?=?itemView總個數(shù)?-?i?=?從最后一個開始
?????????????*
?????????????*?假設?childCount?-?1?=?7
?????????????*?????i累加
?????????????*?????那么level?=?childCount?-?1?-?0?=?7
?????????????*?????那么level?=?childCount?-?1?-?1?=?6
?????????????*?????那么level?=?childCount?-?1?-?2?=?5
?????????????*?????那么level?=?childCount?-?1?-?3?=?4
?????????????*?????那么level?=?childCount?-?1?-?4?=?3
?????????????*??????。。。。
?????????????*/
????????????int?level?=?childCount?-?1?-?i;
????????????if?(level?>?0)?{
????????????????//?最大顯示疊加個數(shù):CardStack2LayoutManager.MAX_SHOW_COUNT?=?4
????????????????if?(level?1)?{
????????????????????//?縮放比例:?CardStack2LayoutManager.SCALE?=?0.05
????????????????????float?scale?=?CardStack2LayoutManager.SCALE;

????????????????????//?valueDip?*?level??=?原始平移距離
????????????????????//?scaleRatio?*?valueDip?=?平移系數(shù)
????????????????????//?valueDip?*?level?-?scaleRatio?*?valueDip?=?手指滑動過程中的Y軸平移距離
????????????????????//?因為是Y軸,所以向上平移是?-?號
????????????????????view.setTranslationY((float)?(valueDip?*?level?-?scaleRatio?*?valueDip));

????????????????????//?1?-?scale?*?level?=?原始縮放大小
????????????????????//?scaleRatio?*?scale?=?縮放系數(shù)
????????????????????//?因為是需要放大,所以這里是?+?號
????????????????????view.setScaleX((float)?((1?-?scale?*?level)?+?scaleRatio?*?scale));
????????????????????view.setScaleY((float)?((1?-?scale?*?level)?+?scaleRatio?*?scale));
????????????????}
????????????}
????????}
????}

????private?int?toDip(Context?context,?float?value)?{
????????return?(int)?TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,?value,?context.getResources().getDisplayMetrics());
????}

滑動系數(shù)圖解:

??:記得綁定?RecyclerView

????//?創(chuàng)建拖拽
????????val?slideCardStackCallBack?=?SlideCardStackCallBack2(cardStackAdapter)
????????val?itemTouchHelper?=?ItemTouchHelper(slideCardStackCallBack)

????????//?綁定拖拽
????????itemTouchHelper.attachToRecyclerView(rootRecyclerView)

這里的注釋比較清晰,來看看最終效果吧~

還有兩個比較好玩的參數(shù)

?//?設置回彈距離
????@Override
????public?float?getSwipeThreshold(@NonNull?RecyclerView.ViewHolder?viewHolder)?{
????????return?0.3f;
????}


????//?設置回彈時間
????@Override
????public?long?getAnimationDuration(@NonNull?RecyclerView?recyclerView,?int?animationType,?float?animateDx,?float?animateDy)?{
????????return?3000;
????}

很簡單,直接看效果

項目地址：https://gitee.com/lanyangyangzzz/android_ui

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這RecyclerView的特效炸了！

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